Toolchains for linux/darwin are always downloaded via a third-party mirror

With the exception of windows, all the other toolchains are currently downloaded from https://github.com/skaji/relocatable-perl . This is potentially an issue as now, even though the rules are authored by this org, this binaries aren't and therefore I have to implicitly trust https://github.com/skaji/relocatable-perl . It would be nice if those archives, like the window one, were mirrored in https://mirror.bazel.build/ .

Inspect the toolchain download definitions for Linux and Darwin and compare them with the Windows toolchain's use of mirror.bazel.build. Identify the archives currently fetched from skaji/relocatable-perl, arrange for corresponding mirror entries, and verify that all supported non-Windows downloads use the Bazel mirror.

Written by the indexing model from the issue text.